September was yet another down month for the stock market, and the outlook still doesn't look great with inflation showing no signs of coming to an end. But if you're a long-term investor, you shouldn't worry about when the markets will recover. As Berkshire Hathaway Vice Chairman Charlie Munger has stated, "To think about what will happen versus when is a far more efficient way to behave."

Taking that advice into consideration, there are three growth stocks that stand out from the rest and that investors should consider buying this month given their promising futures: Eli Lilly (LLY -0.55%), Shopify (SHOP 4.59%), and Costco Wholesale (COST 1.15%).

1. Eli Lilly

Trading at around 50 times earnings, healthcare giant Eli Lilly may turn off some investors with its steep valuation. But earnings multiples don't always reflect long-term growth potential, and that's what Eli Lilly offers. The company's pipeline features more than 20 late-stage trials that could bolster Eli Lilly's financials in the not-too-distant future.

Some analysts believe that its diabetes medicine, Mounjaro, could generate an incredible $25 billion in peak annual revenue. The potential for the treatment to help with weight loss could make it a game changer for the company. Consider that in 2021, Eli Lilly's total revenue from all of its products was $28 billion. With Mounjaro, Eli Lilly could be on track for some serious growth.

This is a business that is going to become a whole lot more valuable and its current share price could look cheap in a few years. How long it takes for Eli Lilly's pipeline to generate significant growth is a question mark, but by focusing on what is likely to happen rather than when, it's a lot easier to make a case for Eli Lilly being a solid growth investment. 

2. Shopify

One of the biggest names in e-commerce is Shopify, whose platform makes it easy for anyone to become a merchant and sell goods and services all over the globe.

With inflation and supply chain challenges recently -- and many consumers strapped for cash -- this is posing a short-term challenge for the business. Sales are still growing, but at $1.3 billion for the period ended June 30, they rose by only 16% year over year -- a far cry from the 57% growth rate Shopify achieved a year earlier.

As a result, Shopify's stock has taken a pounding, falling 80% year to date. But this may be a good example of investors focusing too much on the timeline, and on the current bearish outlook for the economy. In the long haul, e-commerce is still a promising growth opportunity. Analysts from Grand View Research project that the global e-commerce market will expand at a compound annual growth rate of 14.7% through 2027.

With the company now working on cutting costs, that should also position the stock for better gains in the future as its business becomes leaner and more profitable. Investors who focus too much on the short-term results could be missing out. Looking instead on what could happen ahead is what makes the stock a solid, long-term buy.

3. Costco Wholesale

Big-box retailer Costco has been a solid growth stock over the past few years, delivering strong sales numbers at a time when other companies have been struggling.

Through the first nine months of the year, shares of Costco are down 16%, which isn't bad versus the S&P 500, which has declined by 23% over the same time frame. But given the resiliency of the retailer and its ability to generate consistent growth, investors shouldn't expect a huge drop in value from here on out.

Costco reported its fiscal fourth-quarter numbers last month and net sales of $222.7 billion for the fiscal year ended Aug. 28 rose an impressive 16% year over year.

Costco is one of the safest places to invest in right now. Its price-to-earnings multiple of 40 isn't cheap, but given the strength of the business and its ability to draw in customers even with high inflation suggests that it is worth a premium as its success is likely to continue for the foreseeable future.

In Costco's case, the stock is falling because of a bearish outlook on the economy. But if you again focus on what the business is doing well -- drawing in cost-conscious customers during challenging economic conditions -- it's easy to see why Costco's growth can continue and why the stock should remain a better buy than its peers in the long run.